#9210: "overlapping instances" through FunctionalDependencies --------------------------------------------+------------------------------ Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Changes (by rwbarton): * version: 7.8.1 => 7.8.2 Comment: Oh, since I minimized the example now I can test more easily with other GHC versions. This is a regression from GHC 7.6.3, in that that version rejected `main` with the following error (that I don't really understand): {{{ some.hs:22:23: Couldn't match type `Int' with `String' When using functional dependencies to combine Foo (a, x) (a, y) x y, arising from the dependency `a b s -> t' in the instance declaration at some.hs:19:10 Foo (Int, Int) (String, Int) Int String, arising from a use of `foo' at some.hs:22:23-25 In the first argument of `modify', namely `foo' In the second argument of `($)', namely `modify foo (show :: Int -> String) (1 :: Int, 2 :: Int)' Failed, modules loaded: none. }}} If I remove `main` then GHC 7.6.3 does accept the instances, though. That seems wrong to me also (though certainly "less wrong"). I have a GHC 7.8.2.20140609 lying around and it displays the same behavior as GHC 7.8.1. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9210#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler